Correct FS3880 guidance for unsupported language versions

docs/release-notes/.FSharp.Compiler.Service/11.0.200.md

@@ -9,6 +9,7 @@
 * A project reusing cached framework imports is checked with its own `--pathmap` instead of the map of the project that filled the cache, so the ranges of its in-memory reference data name its real files. ([Issue #20474](https://github.com/dotnet/fsharp/issues/20474), [PR #20476](https://github.com/dotnet/fsharp/pull/20476))
 * Split Linux CoreCLR CI into two parallel test batches to reduce test turnaround time. ([PR #20587](https://github.com/dotnet/fsharp/pull/20587))
 * Fix `NativePtr.stackalloc` used inside the `with` handler, filter or `finally` block of a `try` expression producing an assembly that throws `InvalidProgramException` at method load. Direct or mandatory-inline allocations are now reported at compile time as error FS3924; calls to non-inline allocation helpers, including handler-local helpers, remain legal and retain their method boundary during inlining and let-binding elimination. ([Issue #20295](https://github.com/dotnet/fsharp/issues/20295), [PR #20491](https://github.com/dotnet/fsharp/pull/20491))
+* Correct FS3880 guidance in the compiler and F# Interactive to identify .NET SDK 10.0.1xx as the last compatible SDK feature band for out-of-support language versions. ([Issue #20666](https://github.com/dotnet/fsharp/issues/20666), [PR #20795](https://github.com/dotnet/fsharp/pull/20795))
 * Preserve SRTP code generation when extension constraint solutions are disabled. ([PR #20773](https://github.com/dotnet/fsharp/pull/20773))
 * `FsiEvaluationSession.EvalInteraction` and `EvalInteractionNonThrowing` evaluate every `;;`-separated interaction in the text they are given, as their documentation has always said, instead of silently discarding everything after the first. ([PR #20396](https://github.com/dotnet/fsharp/pull/20396))
 * Prevent `InvalidProgramException` when runtime async awaits a `Task<T>` upcast to `Task`, including through `ConfigureAwait`. ([Issue #20686](https://github.com/dotnet/fsharp/issues/20686), [PR #20726](https://github.com/dotnet/fsharp/pull/20726))

src/Compiler/FSComp.txt

@@ -1749,7 +1749,7 @@ featureExtensionConstraintSolutions,"Allow extension members to participate in S
 featureErrorOnBitwiseOpsOnNonIntegralEnums,"Error when bitwise operators are used on enums whose underlying type is not an integer type (e.g. char)."
 featureOptimizeClosureIfNotInlined,"optimize a curried closure argument when its inlining fails"
 featureRequireNamedArguments,"enforce named arguments at call sites of methods annotated with RequireNamedArgumentsAttribute"
-3880,optsLangVersionOutOfSupport,"Language version '%s' is out of support. The last .NET SDK supporting it is available at https://dotnet.microsoft.com/en-us/download/dotnet/%s"
+3880,optsLangVersionOutOfSupport,"Language version '%s' is out of support. The last compatible SDK feature band is .NET SDK 10.0.1xx. Download it at https://dotnet.microsoft.com/en-us/download/dotnet/%s."
 3881,optsUnrecognizedLanguageFeature,"Unrecognized language feature name: '%s'. Use a valid feature name such as 'FromEndSlicing'."
 3882,lexHashElifMustBeFirst,"#elif directive must appear as the first non-whitespace character on a line"
 3883,lexHashElifMustHaveIdent,"#elif directive should be immediately followed by an identifier"

tests/FSharp.Compiler.ComponentTests/CompilerOptions/fsc/langversion.fs

@@ -8,6 +8,39 @@ open FSharp.Test.Compiler
 
 module langversion =
 
+    [<Theory>]
+    [<InlineData("4.6")>]
+    [<InlineData("4.7")>]
+    [<InlineData("5")>]
+    [<InlineData("5.0")>]
+    [<InlineData("6")>]
+    [<InlineData("6.0")>]
+    [<InlineData("7")>]
+    [<InlineData("7.0")>]
+    let ``Issue 20666 - fsc unsupported langversion identifies the last compatible SDK feature band`` (version: string) =
+        let expected =
+            $"Language version '{version}' is out of support. The last compatible SDK feature band is .NET SDK 10.0.1xx. Download it at https://dotnet.microsoft.com/en-us/download/dotnet/10.0."
+
+        FSharp """printfn "Hello, World" """
+        |> asExe
+        |> withOptions [$"--langversion:{version}"]
+        |> compile
+        |> shouldFail
+        |> withErrorCode 3880
+        |> withDiagnosticMessage expected
+        |> ignore
+
+    [<Theory>]
+    [<InlineData("default")>]
+    [<InlineData("latest")>]
+    let ``fsc langversion supports named selectors`` (version: string) =
+        FSharp """printfn "Hello, World" """
+        |> asExe
+        |> withOptions [$"--langversion:{version}"]
+        |> compile
+        |> shouldSucceed
+        |> ignore
+
     [<Fact>]
     let ``fsc langversion is case-insensitive - --langversion:pRevIew``() =
         FSharp """

tests/FSharp.Compiler.ComponentTests/CompilerOptions/fsi/Langversion.fs

@@ -11,6 +11,44 @@ open FSharp.Test.Compiler
 
 module Langversion =
 
+    [<Theory>]
+    [<InlineData("4.6")>]
+    [<InlineData("4.7")>]
+    [<InlineData("5")>]
+    [<InlineData("5.0")>]
+    [<InlineData("6")>]
+    [<InlineData("6.0")>]
+    [<InlineData("7")>]
+    [<InlineData("7.0")>]
+    let ``Issue 20666 - fsi unsupported langversion identifies the last compatible SDK feature band`` (version: string) =
+        let expected =
+            $"Language version '{version}' is out of support. The last compatible SDK feature band is .NET SDK 10.0.1xx. Download it at https://dotnet.microsoft.com/en-us/download/dotnet/10.0."
+
+        let result = runFsiProcess ["--nologo"; "--exec"; $"--langversion:{version}"]
+        Assert.NotEqual(0, result.ExitCode)
+        Assert.Contains("error FS3880:", result.StdErr)
+        Assert.Contains(expected, result.StdErr)
+        Assert.DoesNotContain("10.0.2", result.StdErr)
+        Assert.DoesNotContain("10.0.4", result.StdErr)
+
+    [<Theory>]
+    [<InlineData("8")>]
+    [<InlineData("8.0")>]
+    [<InlineData("9.0")>]
+    [<InlineData("10.0")>]
+    [<InlineData("11.0")>]
+    [<InlineData("11.2")>]
+    [<InlineData("default")>]
+    [<InlineData("latest")>]
+    [<InlineData("latestmajor")>]
+    [<InlineData("preview")>]
+    [<InlineData("pRevIew")>]
+    [<InlineData("laTestmaJor")>]
+    let ``fsi langversion supports numeric versions and named selectors`` (version: string) =
+        let result = runFsiProcess ["--nologo"; "--exec"; $"--langversion:{version}"]
+        Assert.Equal(0, result.ExitCode)
+        Assert.DoesNotContain("FS3880", result.StdErr)
+
     // Test: --langversion:4.5 - unrecognized version
     // Original: SOURCE=badlangversion.fsx SCFLAGS=" --langversion:4.5"
     [<Fact>]